Project summary: Exposure to environmental chemicals is a major health risk. Unfortunately, the detrimental impacts of toxin exposure vary among individuals in a population because of unknown genetic differences. With a better understanding of how our genetics influence toxin response, we can more accurately predict detrimental health effects. It is difficult to identify these factors because human genome-wide association studies often lack the necessary statistical power and controlled toxin exposures. For this reason, we will use defined population-wide variation in the roundworm Caenorhabditis elegans to enable precise measurements of toxin responses at the scale and statistical power of single-cell organisms but with conserved molecular, cellular, and developmental properties of a metazoan. In Aim 1, we will identify genetic loci underlying variation in response to 30 diverse toxins, including metals/metalloids, mitochondrial toxins, pesticides, and flame retardants. We will define effective toxin doses across diverse individuals using low-cost, high-throughput, and high-accuracy assays of growth and fertility. Then, we will define the population-wide variation in response to these 30 toxins and use these data to map toxin-response differences to genes using two mapping panels: (1) CeNDR - the C. elegans Natural Diversity Resource, a set of 500 strains representing nearly all known genetic variation for the species, and (2) CeMEE - the C. elegans Multiparental Experimental Evolution panel, a set of 1000 recombinant inbred lines that enable mapping to the resolution of single genes. In Aim 2, we will identify specific genetic variants and pathways affecting toxin-response variation. We will define causal relationships between toxin response differences and genetic variants using state-of-the-art breeding and genome-editing techniques. Then, we will use gene expression analyses and hypothesis- directed experiments to determine the molecular basis of toxin-response variation. In Aim 3, we will elucidate conserved mechanisms of toxin-response variation by mapping toxin responses in two other Caenorhabditis species that are as genetically different from each other as mice and humans. An innovative comparative quantitative trait locus analysis will ensure identification of sources of toxin-response variation that arise convergently (and therefore predictably) in multiple evolutionary lineages. We will extend this approach by further comparing our mapping results to those from Drosophila, rodents, and humans, identifying conserved pathways responsible for toxin-response variation. Our Caenorhabditis genetic resources have levels of variation, allele frequencies, and phenotypic effects similar to humans, providing a framework to discover the characteristics of genes and variants that underlie differences in human toxin responses. Indeed, decades of research in C. elegans have identified countless examples of widely conserved molecular mechanisms underlying signaling, gene regulation, and metabolism, suggesting that the toxin-response mechanisms discovered here will extend to humans despite overt differences in life history and anatomy.